The present invention generally relates to reflection-type liquid crystal display devices used in a low-power apparatuses such as portable terminals.
A reflection-type liquid crystal display device is a liquid crystal display device that achieves display of images by incorporating environmental light such as interior illumination light or sunlight and causing the same to reflect toward an observer by means of a reflector.
Because of the operational principle, the reflection-type liquid crystal display device does not need a backlight and has an advantageous feature of low power consumption. Thus, reflection-type liquid crystal display devices are used extensively for portable terminals.
In order to achieve bright and clear representation of images in a reflection-type liquid crystal display device, it is necessary to design the liquid crystal display device such that as much environmental light as possible is incorporated and reflected toward the observer in the white representation mode and that the reflection of the incorporated light toward the observer is suppressed as much as possible in the black representation mode.
Thus, there is a proposal of a reflection-type liquid crystal display device that uses a phase-change type guest-host (GH) mode (D. L. White and G. N. Taylor, J. Appl. Phys. 45, pp.4718, 1974). Because a GH-mode reflection-type liquid crystal display device does not require a polarizer, there is a distinct advantage in such a GH-mode reflection-type liquid crystal display device that a bright representation is achieved in the white representation mode.
On the other hand, a GH-mode liquid crystal display device has a drawback in that a bright representation is obtained also in the back representation mode and the contrast ratio is limited to the range of 5-6.
Meanwhile, there is a proposal of a reflection-type liquid crystal display device of a twisted nematic mode that uses a single polarizer as in the Japanese Laid-Open Patent Publication 6-11711.
This conventional reflection-type liquid crystal display device is basically a horizontally oriented liquid crystal device in which liquid crystals having a positive dielectric anisotropy are twisted. In the foregoing conventional reflection-type liquid crystal display device, the incoming environmental light is converted to a linear polarization light by a polarizer, and the linearly polarized light thus obtained is passed through a liquid crystal layer or a phase compensation film having a ¼-wavelength retardation, so that there is achieved a 90 degree angle of polarization plane between the incident light passed through the polarizer and the reflection light returning to the polarizer.
Thus, in this conventional liquid crystal display device, the black representation is achieved by absorbing the rotated reflection light by the polarizer. Because of the use of the polarizer, the foregoing conventional liquid crystal display device can provide only about 40% of brightness in the while representation mode as compared with the case of the GH-mode liquid crystal display device. However, the liquid crystal display device can achieve a contrast ratio of 12-14 in view of efficient absorption of the light in the black representation mode.
Further, there is a proposal of improving the contrast ratio in a TN-mode liquid crystal display device by way of compensating for the black representation, by reducing the amount of retardation of the phase compensation film by the magnitude of the residual retardation of the liquid crystal layer. See Japanese Laid-Open Patent Publication 11-311784. With this, the contrast ratio is improved to about 16-18.
In a reflection-type liquid crystal display device, the visibility of representation is defined by brightness and contrast ratio. Thus, a high visibility is achieved even in the case of low contrast ratio when the representation is bright. When the representation is dark, on the other hand, a large contrast ratio is required. See The Journal of the Institute of Television Engineers of Japan, Vol.50, No.8, pp.1091-1095 (1996).
A contrast ratio of about 12 is needed in order to realize the visibility comparable to that of a GH-mode liquid crystal display device by using a liquid crystal display device having a single polarizer, the latter liquid crystal display device can provide the brightness of only 40% of the brightness of a GH-mode liquid crystal display device. By using the technology noted in the above reference, it becomes possible to achieve a contrast ratio of 16-18 by using a T-N mode liquid crystal display device.
Because of the foregoing reason, and further in view of better reliability, a TN-mode liquid crystal display device having a single polarizer is used widely in these days for a reflection-type liquid crystal display device.
In a TN-mode liquid crystal display device having a single polarizer, it should be noted that the upper and lower substrates are subjected to rubbing processing in different directions so as to realize the twisted structure in the liquid crystal layer. As a consequence, the anchoring direction of the liquid crystal layer is not coincident in the upper and lower substrates.
Because of this, the technology of the foregoing Japanese Laid-Open Patent Publication 11-311784 sets the retardation axis of the phase compensation film at the angle intermediate between the upper and lower anchoring directions so as to compensate for the synthetic vector of the upper and lower anchoring directions. However, this construction cannot compensate for the residual retardation of the liquid crystal layer at the upper and lower substrates individually and the compensation for the black representation remains incomplete.
Meanwhile, there is a proposal of a reflection-type liquid crystal display device of vertically aligned (VA)-mode that uses a single polarizer (See Japanese Laid-Open Patent Publication 6-337421).
In such a VA-mode liquid crystal display device, the On and Off operation is just the opposite as in the case of a TN-mode liquid crystal display device. On the other hand, the operational features of: converting the incoming environmental light to linearly polarized light by the polarizer; rotating the polarization plane of the linearly polarized light thus obtained by 90 degrees by using a liquid crystal layer or a phase compensation film having a retardation of about ¼ wavelength of visible light; and causing the polarizer to absorb the rotated linearly polarized light in the black representation mode, are identical between the foregoing VA-mode liquid crystal display device and the TN-mode liquid crystal display device.
On the other hand, the VA-mode reflection-type liquid crystal display device is advantageous in the point that there remains no liquid crystal layer causing anchoring at the liquid crystal/substrate interface in the black representation mode contrary to the case of the TN-mode liquid crystal display device because of the fact that the black representation mode is achieved in the VA-mode reflection-type liquid crystal display device in the state no voltage is applied to the liquid crystal layer. Thereby, the contrast ratio of image representation is improved significantly.
In this way, the VA-mode reflection-type liquid crystal display device has an advantageous feature of high contrast ratio and excellent visibility.
On the other hand, there still exist problems to be solved in such a VA-mode reflection-type liquid crystal display device particularly with regard to the control of alignment of the liquid crystal molecules.
More specifically, a VA-mode liquid crystal display device generally uses a vertical alignment film, while the performance of such a vertical alignment film may be degraded seriously when subjected to a rubbing process. For example, there may be caused defective display of images such as uneven brightness extending in the form of streaks
Because of this reason, there is a need of achieving alignment control of liquid crystal molecules in a VA-mode liquid crystal display device by means other than rubbing.
In the Japanese Laid-Open Patent Publication 10-301112, for example, the alignment control of the liquid crystal molecules is achieved by providing a slit extending obliquely in a reflection electrode on the opposite substrate, such that there is induced an oblique electric field between the upper and lower substrates upon application of a voltage.
This technology, on the other hand, has a drawback in that the overall reflectivity of the pixels is reduced because the part of the liquid crystal layer located immediately on the slit does not undergo switching and the visibility of the image representation is not very much improved even when the contrast ratio is improved.
Thus, there has been a need of improving the contrast ratio without sacrificing the reflectivity in a VA-mode liquid crystal display device.
Meanwhile, a reflection-type liquid crystal display device generally has a problem of the visibility influenced heavily by the optical environment such that the visibility of the images is degraded seriously in the dark optical environment. With this respect, a transmission-type liquid crystal display device having a backlight provides far superior visibility. On the other hand, a transmission-type liquid crystal display device suffers from the problem of poor visibility in the bright optical environment in that the obtained visibility is inferior to the visibility achieved by the reflection-type liquid crystal display device.
Thus, in order to improve the foregoing problems, there has been proposals such as using a front light in combination with a reflection-type liquid crystal display device, or a reflection-type liquid crystal display device having a semi-transparent reflection film.
The approach of using a front light, however, suffers from the problem in that the contrast ratio achieved in the dark optical environment may be inferior to the contrast ratio of the direct-view type transmission-type liquid crystal display device. In the bright optical environment, on the other hand, there may arise another problem in that the representation becomes dark as compared with the conventional reflection-type liquid crystal display device because of the existence of the front light.
In the case of using a semi-transparent film, a metal thin film is generally used for this purpose. However, a metal thin film has a large absorption coefficient and has a problem in the efficiency of utilization of light. Further, the metal thin film suffers from the problem of conspicuous variation of transmittance because of the in-plane variation of film thickness. It should be noted that such a metal thin film is generally provided by a thin Al film having a thickness of about 30 nm. Currently, it is difficult for form a metal thin film with uniform thickness over a wide display area.
In order to eliminate the foregoing problem, there has been a proposal in the Japanese Laid-Open Patent Publication 11-281972 in which there is provided a transparent window by means of a transparent electrode such as ITO (In2O3.SnO2) at the central part of the pixels. According to this conventional proposal, the foregoing problems are eliminated and it became possible to construct a reflection-transmission-type liquid crystal display device.
On the other hand, the foregoing conventional proposal of the reflection-transmission-type liquid crystal display device has needed formation of projections and depressions on a planarized film and formation of a step in the transmission region by forming a hole. Further, the foregoing technology requires formation of both the transparent electrode (ITO) and the reflection electrode (Al) and further the formation of a barrier metal film for preventing electrolytic corrosion, which may be caused at the contacting part of the Al pattern and the ITO pattern. Thus, the fabrication process of the liquid crystal display device is complex and the cost of fabrication could not be reduced.
Further, the conventional reflection-type liquid crystal display device, relying upon the principle of optical switching caused by retardation of the liquid crystal layer, has to be designed to have a cell thickness of ½ of the wavelength of the visible light in the transmission region and a cell thickness of ¼ of the wavelength of the visible light in the reflection region. However, such a structure has been difficult to produce.